1. Field of the Invention
The present invention relates to an apparatus and process for forming a Bragg grating in an optical fiber element. More particularly, the present invention relates to an apparatus and a continuous or stepwise continuous process for making optical fiber Bragg gratings in a coated optical fiber element. In a presently preferred embodiment, the process of the present invention includes the steps of removing a sufficient amount of a removable coating from at least one predetermined section of an optical fiber element such that optical radiation may access a core of the optical fiber, immobilizing the predetermined section of the optical fiber, forming at least one Bragg grating in the predetermined section of the optical fiber, and treating the predetermined section of the optical fiber to package the Bragg grating.
2. Description of Related Art
A Bragg diffraction grating is a structure that has a periodic pattern of alternating high and low optical refractive index values. Bragg gratings are useful because of their ability to reflect a particular wavelength or "color" of light. The color that will be reflected by a grating is the color whose wavelength exactly matches twice the effective grating period. See, for example, Morey et al., Photoinduced Bragg Gratings in Optical Fibers, Optics and Photonics News, vol. 5, no. 2 (February 1994); Meltz et al., Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method, Opt. Lett. 14 (1989) at 823-25.
It is well known that Bragg gratings may be formed by creating an interference pattern in the germanosilicate glass core of an optical fiber, typically by recombining two parts of the beam of an ultraviolet laser. The first optical fiber Bragg gratings were produced accidentally when an argon ion laser remained focused into the end of an optical fiber for a period of hours. A portion of the beam was reflected back upon itself in the fiber, producing a standing wave interference pattern. In the bright sections of the interference pattern (where the forward- and backward-traveling waves reinforce each other), the laser light interacted with germanium sites in the fiber core and changed the local refractive index. At the dark sections of the interference pattern (where the two waves destructively interfere and cancel each other), the refractive index remained unchanged.
However, this "end launch" method of "writing" Bragg gratings in optical fibers allows almost no control of the location of the grating within the fiber, the angle of the grating planes with respect to the optical fiber axis, or the grating period. All of these variables are important to control when constructing useful devices based on fiber optic Bragg gratings, and the end launch method has not proved useful for producing optical fiber Bragg gratings in commercial quantities.
To provide greater flexibility in the design of fiber optic Bragg grating devices, techniques have been developed to write gratings by applying optical radiation through the side of (e.g. normal to the length of) an optical fiber. One such technique, as illustrated in U.S. Pat. Nos. 4,725,110 and 4,807,950, involves splitting a laser beam into two sub-beams and recombining these sub-beams at a known and controllable angle within the core of the optical fiber. A second well-known technique described in the technical and patent literature involves focusing the laser beam on the fiber core through a grooved or patterned transmissive optical element known as a phase mask. This phase mask holographically creates an interference pattern in the optical fiber core.
The above-described techniques for producing optical fiber Bragg gratings are well established, but certain technical difficulties to date have prevented their use in large scale continuous or stepwise continuous production processes. For example, a significant production problem is removal of the coating which covers the section of the optical fiber to be treated with the laser. Optical fibers are produced with a coating which protects the delicate glass structure from chemical or mechanical attack, and this coating must be substantially completely removed if the applied optical radiation is to access and form a Bragg grating in the optical fiber core. If a coated optical fiber is to be used in the manufacture of a fiber Bragg grating, it is necessary first to thermally, chemically or mechanically remove all or a part of the protective coating from the coated optical fiber to leave an optically treatable, preferably bare, fiber surface. See, for example, Rizvi and Gower, Production of Bragg Gratings in Optical Fibers by Holographic and Mask Production Methods, The Institute of Electrical Engineers, Optical Fiber Gratings and Their Applications, January 1995.
However, conventional thermal, mechanical or chemical means for stripping the coating from the bare fiber in manufacturing processes are time consuming and reduce the physical integrity of the fiber. See, e.g., M. C. Farries et al., Fabrication and Performance of Packaged Fiber Gratings for Telecommunications, The Institute of Electrical Engineers, Optical Fiber Gratings and Their Applications, January 1995; Tang et al., Annealing of Linear Birefringence in Single-Mode Fiber Coils: Application to Optical Fiber Current Sensors, Journal of Lightwave Technology, vol. 9, No 8, August 1991. Therefore, careful removal of the optical fiber coating is required to form a sufficiently clean glass surface to allow treatment of the optical fiber core with the laser, as well as an optical fiber which retains its strength after formation of the Bragg grating in the core. Time-consuming and labor intensive coating removal steps have to date limited manufacture of optical fiber Bragg gratings to production in small batches. In these batch processes the coating is typically chemically removed from a short length (referred to herein as a "section") of several optical fibers. The fibers are then treated, one at a time, with a laser using a phase mask projection technique to form Bragg gratings in the sections of the optical fibers where the coating was removed. These production processes provide good control over formation of a single Bragg grating in a short length of optical fiber. However, the batch technique is obviously not economically feasible for large scale Bragg grating production, or for production of multiple Bragg gratings in a long length of optical fiber for grating arrays. In addition, in the batch technique the bare optical fiber is exposed for significant lengths of time, which may degrade fiber strength. To monitor grating quality, the batch technique requires a termination for each optical fiber end.
To address the coating removal problems in the batch production technique, some optical fiber Bragg gratings have been written as the optical fiber is produced on the draw tower. Draw tower production makes coating removal unnecessary, since the optical fiber cores are treated with optical radiation to form Bragg gratings before their protective coating(s) is applied. Formation of Bragg gratings during fiber draw increases production volume compared to the batch process described above. However, as the optical fibers are drawn on the draw tower, the Bragg gratings must be formed with a single shot from the laser, and the draw process cannot be stopped or interrupted to use different grating writing techniques. Further, the Bragg condition (for example, center band wavelength) of the Bragg grating depends on the exact placement of a predetermined section of the optical fiber relative to a writing zone, and since the position of the optical fiber drawn on the tower cannot be precisely controlled, the grating writing process cannot be sufficiently stable from shot to shot. The variation in draw speed also makes precise location of the Bragg grating difficult. Therefore, while the draw tower production technique increases production speed compared to the batch process, this speed comes at a significant cost in grating quality and precision.
To date, no apparatus or process for the large scale manufacture of optical fiber Bragg gratings has been identified which provides production speed and efficiency, ensures grating quality, and maintains optical fiber strength following grating formation.